A TAAH aqueous solution is a useful chemical as a strong base in a chemical reaction and has been recently used also as a processing chemical in a semiconductor-related field, such as the cleaning and etching of a semiconductor substrate and the development of a resist in the production of ICs and LSIs.
For use as a processing chemical in the semiconductor-related field, a high-purity tetraalkylammonium hydroxide aqueous solution containing no impurities such as metal ions and organic substances has been desired, along with progress made in the integration of semiconductor devices.
Heretofore, the above TAAH aqueous solution has been produced by dissolving solid tetraalkylammonium chloride (may be abbreviated as TAAC hereinafter) in water to prepare a TAAC aqueous solution and electrolyzing and electrodialyzing the TAAC aqueous solution to effect ion-exchange between chlorine ions and hydroxyl ions.
In the above production method of the TAAH aqueous solution, reasons why TAAC is used in a solid state are as follows. That is, TAAC has been heretofore produced by reacting a trialkylamine with an alkyl chloride in a polar solvent such as water, isopropanol or the like. However, in the industrial production, water as a solvent has been rarely used actually and an organic solvent such as isopropyl alcohol has been generally used.
Therefore, to prepare a TAAC aqueous solution, there is employed a method in which solid TAAC obtained by drying a TAAC aqueous solution to remove the solvent is dissolved in water. This is one of the reasons for handling TAAC in a solid state.
Another reason is that TAAC is generally used as an additive such as a phase-transfer catalyst or reagent and it must be solid when it is directly added to a reaction system.
Still another reason is that solid TAAC is advantageous in transportation between factories, handling and the like.
However, the above-mentioned application involves problems. That is, the method of producing a TAAH aqueous solution from the above solid TAAC requires a process of drying TAAC into a solid. Therefore, concentrated metal impurities such as metal ions derived from a solvent and raw materials are contained in TAAC, and further, when TAAC is dissolved in water, impurities contained in water are added thereto. Consequently, the purity of the obtained TAAH aqueous solution is greatly lowered.
Further, at the time of drying, TAAC is partially decomposed, whereby the yield thereof is lowered and at the same time, the purity of the obtained TAAH aqueous solution is lowered by the decomposition product, like in the above case.
Meanwhile, use of purified TAAC as a raw material of the TAAH aqueous solution to improve the purity of the TAAH aqueous solution is known as disclosed in Japanese Laid-open Patent Application 60-131985 (131985/1985), for example. The above publication teaches that the impurities of alkali metals and alkaline earth metals out of impurities derived from raw materials can be reduced to such an extent that a required purity of a quaternary hydroxide can be obtained by selecting a quaternary ammonium salt as a raw material with care and purifying it.
However, TAAC cannot be purified by distillation due to its properties, and the industrial-scale purification thereof is limited to a separation method in an ion state such as electrodialysis. For example, when are purification is conducted by letting quaternary ammonium ions passing through a cation exchange membrane, most of alkali metal ions pass through a separating membrane together with tetraalkylammonium ions in the purification because alkali metal ions have a smaller ion diameter than tetraalkylammonium ions. Meanwhile, for removing alkali metal ions by a univalent perm-selective membrane completely, it takes extremely long time and hence, this is not practical.
Therefore, it has been difficult to purify TAAC to such an extent that the concentration of alkali metal ions is several ppb, which is the target of the present invention, with the conventional method.
The purification by electrodialysis of the obtained TAAH aqueous solution is disclosed also in Japanese Laid-open Patent Application 60-131985. According to this publication, a means to purify the TAAH aqueous solution containing the above impurities comprises supplying the TAAH aqueous solution to be purified to an anode chamber formed by installing a cation exchange membrane between an anode and a cathode, supplying water to a cathode chamber and applying electricity between the both electrodes to obtain purified TAAH from the cathode chamber.
However, in this case, too, the removal of univalent ions such as sodium and potassium is not sufficient like the purification of the aforesaid TAAC aqueous solution and hence, an extremely high-purity TAAH aqueous solution does not yet come to be obtained.
As described above, in any of the above conventional methods for obtaining a high-purity TAAH aqueous solution, purification is carried out in a condition that separation of metal ions such as sodium and potassium is difficult to conduct, and the level of high purity thereof is required to be further improved.